Our ferrets were doing this too.  The last thing we wanted was for them to
get up in there and one of us sit on the couch and kill them.  I went to
Walmart, bought the heaviest, cheapest fabric I could find large enough to
cover the entire bottom of the couch.  I then tacked it to the frame.  No
one is ever going to see it, so it did not matter what the material looked
like.  It has been about 2 years since we did this, and they have yet to
dig into the couch again.  And believe me, they have tried!
 
Before you tack on the new fabric, make sure you tear off all the old
stuff and check inside the couch.  We found that Stinky had been spending
more time in there than we thought!  He had stashed several prizes in
there!
 
Lisa
Stinky, Pepper, Mitsy and Pristine
[Posted in FML issue 3582]